In some neoplastic diseases, particularly gynaecological cancers like breast cancer, the response to neoadjuvant chemotherapy is comparatively low, with only about 20% of patients achieving pathological complete remission (pCR) with no tumor cells left in the breast or lymph nodes; the latter being the strongest prognostic factor for prolonged survival due to treatment benefit to date.
However, a substantial number of patients suffer severe side effects (ADRs) from highly toxic drug combinations (e.g. alopecia due to inclusion of taxanes) without additional benefit. In addition, there is a burden on national health systems due to the high cost of some therapies in this regime, especially if the chemotoxic treatments are combined with new targeted treatment options (e.g. Herceptin®, Lapatinib® and Avastin®). Moreover the new treatment options are related with some severe, probably life threatening side effects (e.g. cardiac toxicities upon combinatorial treatment with Herceptin®, gastrointestinal perforation upon combinatorial treatment with Avastin).
A better characterization of the respective tumors would thus allow a better selection of the most promising therapy in a given breast cancer patient, in order to avoid unnecessary side effects due to neoadjuvant chemotherapy in those patients which do no not draw any benefit from such therapy anyway.
Some neoplastic diseases, particularly gynaecological cancers like breast cancer (BC), are characterized the fact that approximately 80% of them are estrogen receptor positive as characterized by standard immunohistochemistry, i.e. the exhibit estrogen receptors. However, it turned out that only a fraction of these tumors are dependent on hormone ligands (i.e. estrogen) for activation of Estrogen receptors (ESR) and sustained growth of the tumor tissue.
The estrogen receptor is a member of the nuclear hormone family of intracellular receptors which is activated by the hormone 17-β-estradiol (estrogen). The main function of the estrogen receptor is that of a DNA binding transcription factor which regulates gene expression. In addition a subfraction of estrogen receptor is able to interact with receptor tyrosine kinases (e.g. Her-2/neu) on the membrane which is critical for development of resistance towards cancer therapeutics. Estrogen and the ESRs have also been implicated in breast cancer, ovarian cancer, colon cancer, prostate cancer and endometrial cancer. Advanced colon cancer is associated with a loss of ERβ (also termed ESR2), the predominant ESR in colon tissue, and colon cancer is treated with ERβ specific agonists in some cases.
As stated above, Estrogen receptors are overexpressed on the protein level in around 80% of breast cancer cases, referred to as “ESR positive”. Two hypotheses have been proposed to explain why this causes tumorigenesis. One stipulates that binding of estrogen to the ESR stimulates proliferation of mammary cells, with the resulting increase in cell division and DNA replication leading to mutations. The other one states that estrogen metabolism produces genotoxic waste.
The result of both processes is disruption of cell cycle, apoptosis and DNA repair and therefore tumor formation or growth.
Different versions of the ESR1 (also termed ERα), gene have been identified (with single-nucleotide polymorphisms) and are associated with different risks of developing breast cancer.
It has turned out that, typically, ESR-positive tumors demonstrate only poor responses on neoadjuvant chemotherapy, with about 10% pathological complete remission (pCR) reported.
However, ESR-positive tumors may profit from a treatment with Tamoxifen, an estrogen-receptor antagonist used as an adjuvant hormonal treatment. Another selective estrogen receptor modulator, raloxifene, has been used as a preventative chemotherapy for women judged to have a high risk of developing breast cancer. Another anti-estrogen, ICI 182,780 (Faslodex) which acts as a complete antagonist also promotes degradation of the estrogen receptor.
Other anti estrogen drugs are Anastrozole (Arimidex®), a drug which prevents the conversion of adrenal gland androgen hormones to estrogen, Exemestane (Aromasin®) and Letrozole (Femara®), which are inhibitors for the enzyme aromatase which is involved in the production of estrogen, and Megestrol acetate (Megace®) which is a progesteron agonist acting trough competitive inhibition.
One current standard for diagnosis of early breast cancer is the determination of ESR1 by immunohistochemistry (IHC) using subsequent scoring systems. These assays are based on Protein-level measurements exhibiting limited quantitative performance and comparatively high inter- and intra-assay variabilities. Moreover, the final assessment is essentially subjective and is known to show substantial inter-operator (i.e. inter-pathologist) variance (Faneyte et al., 2003).
In this context, it has been shown that as few as 1 to 5% of ESR1 positive tumor cells within a given tumor are sufficient to specify this tumor as being potentially responsive to endocrine treatment. This is somewhat surprising as one would rather think that the 95% to 99% ESR1 protein negative tumor cells should not be dependent on estrogen and thereby not be responsive to endocrine treatment as described above. Conversely, this already shows the limitations of the protein determination of estrogen receptors as being insufficient to describe estrogen receptor dependent tumors. Clinically the 95% to 99% of estrogen receptor negative tumor cells have a high potential to be hormone dependent. Moreover, the determination of estrogen receptor status based on immunohistochemistry is highly subjective and varies between different labs (approximately 70% concordance). In view, of the marginal protein expression level of estrogen receptor being necessary to qualify for endocrine treatment this is critical.
Moreover, there are apparent differences between ESR1 positive tumors, which clearly separate the growth characteristics and dependency on solely estrogen. For example, it has been shown that a significant fraction of estrogen receptor coexpress progesteron receptor and/or the receptor tyrosine kinase Her-2/neu. This raises e.g. the possibility of estrogen independent growth capabilities via progesterone or EGFR family ligands.
Nevertheless, Estrogen receptor positive tumors do have a comparably good prognosis, while Estrogen receptor negative tumors as determined by IHC have a particularly bad prognosis.
It has yet been reported that about 20% of breast cancer cases are independent of estrogen, and are thus resistant against anti estrogen treatments (Ring et al., 2004).
These tumors, however, seem to demonstrate a better response towards chemotherapy, with about 20% pathological complete remission (pCR) reported. In addition, if Her-2/neu positive, these tumors may additionally have benefit from anti-Her-2/neu regimen such as Herceptin™ or Tykerb™. Apparently, bad prognosis tumors particularly bear the potential of benefit from combined antibody and chemotherapeutic regimen.
Still, not at least in view of the new therapeutic options, the worst prognosis among the breast cancer subgroups do have estrogen receptor negative, progesterone receptor negative and Her-2/neu receptor negative breast cancer, which are also the so called “basal like tumors” as originally defined by multiparametric gene array analysis by unsupervised cluster analysis (Sorlie et al., 2001)
However, the precise definition of the so called “basal like tumors” has been defined by fresh tissue RNA analysis using multigene arrays, and the definition of the “basal like tumors” by immunohistochemistry in fixed tissue routine samples is far from being adequate. Moreover, the “basal like tumors” itself seem to be clinically heterogenous and do contain two very different subtypes, one of which seems to have a particularly good response to chemotherapy.
A proper differentiation between these two “basal like” tumor subclasses would help to apply or develop patient or tumor specific therapies, in order to reduce side effects and improve tumor remission rates.
Moreover, new targets for newly available targeted drugs, or drugs yet to be developed, could thus be determined.
It is obvious that current methods do not suffice to characterize a high risk or low risk “basal like tumor” in a reliable and reproducible way by immunohistochemically determining it as ESR-negative, PR-negative and Her-2/neu negative.